Plastic substrates are commonly used in automotive parts and accessories. Organic coating compositions are very often applied to these substrates for decorative and protective purposes. These plastic substrates are made of a variety of flexible thermosetting and thermoplastic materials such as polyethylene and polypropylene, thermoplastic urethane, polycarbonate, thermosetting sheet molding compound, reaction-injection molding compound, acrylonitrile-based materials, nylon and the like. The coating compositions that are used on these substrates must also be flexible so as to avoid cracking and adhesive failure under normal stresses and torsional forces to which the substrates may be subjected.
Color-plus-clear coating systems involving the application of a colored or pigmented base coat to a substrate followed by the application of a transparent or clear topcoat to the base coat have become very popular as original finishes for automobiles. The color-plus-clear systems have outstanding gloss and distinctness of image and the clear coat is particularly important for these properties.
Coating systems of the prior art which are known to be resistant to acid etch include acid-epoxy curable compositions such as those disclosed in U.S. Pat. No. 4,681,811 and compositions containing hydroxyl functional polymers reacted with isocyanates or polyisocyanates to form polyurethanes. The isocyanates are expensive and the toxicity of the isocyanates is an additional drawback.
Coating systems that employ hydroxy-aminoplast cure mechanisms are well known in coating technology and provide many excellent coating properties. They are inexpensive, durable and attractive. However, it is widely recognized that such coatings, particularly clear coats, have poor resistance to etching by acid. Acid etch resistance in coatings is becoming an increasingly desirable property, particularly for automotive coatings. Aminoplast cured coating systems of the prior art are not highly effective for providing protection against etching caused by acid rain. Additionally, such coating systems are often "high solids" coating compositions which tend to sag upon application to vertical surfaces and during baking. Such sagging adversely affects the appearance properties of the coating compositions.
It is desirable to provide a coating composition having improved appearance and performance properties, such as sag and acid etch resistance, utilizing inexpensive aminoplast technology. It is also desirable that such coating compositions be flexible enough for use on flexible automotive substrates and suitable for use in a color-plus-clear composite coating system.